Microwave frequency divider



Jan. 30, 1962 D. A. DUNN 3,019,366

MICROWAVE FREQUENCY DIVIDER Filed July 29, 1958 Hill Ld' L3 INVENTOR, DONALD A. DUNN ATTORN EYL rates This invention relates to microwave frequency dividers and more particularly to microwave frequency dividers utilizing backward-wave type travelling-wave tubes.

Recent developments in frequency divider circuits operable at microwave frequencies have produced a forward travelling-wave type tube wherein frequency division results from the non-linear characteristics of the electron stream. One such frequency divider is described on pages 10131015 of the July 1957 issue of the Proceedings of the IRE. While such tubes were found to operate reasonably well as frequency dividers in the microwave range region, they required an external feedback path which, in turn, introduced frequency sensitivity. For proper operation, such forward-wave travelling-wave tube dividers required an adjustment of the helix voltages to obtain the necessary phase relationship.

It is therefore an object of the present invention to provide a microwave frequency divider wherein the abovementioned limitations are overcome.

It is another object of the present invention to provide an improved microwave frequency divider which is inherently narrow-band.

It is still another object of the present invention to provide an improved mircowave frequency divider wherein the phase angle of the feedback is always optimum and is not a function of frequency. In brief, the microwave fre quency divider circuit includes a travelling-wave tube having an electron gun at one end thereof for directing an electron beam along a path and a collector anode at.

the other end thereof for intercepting the beam. Also included are means including a first slow-wave circuit element in coupling relationship to the beam and having a signal input means at the end thereof nearer the collector anode and an energy absorber at the other end thereof, whereby when a signal of prescribed frequency and power is applied to the signal input means, the beam is saturated and interacts with the signal in the backward-wave mode such that the beam is modulated at the input frequency. Included further is a second slow-wave circuit element intermediate the first slowwave circuit element and the collector anode and in coupling relationship to the beam. The second slow-wave circuit element is provided with a signal output means at the end thereof nearer the electron gun and an energy absorber at the other end thereof, and the beam current in coupling relation thereto is below the start-oscillation current. In addition, there is included means for voltage tuning the second slow-wave circuit element whereby backward wave interaction between a signal propagated in the second slow-wave circuit element and the beam in coupling relation thereto produces at the output thereof a signal which is a submultiple of the prescribed frequency only when the beat frequency between the submultiple frequency and the prescribed frequency in the beam is equal to the submultiple frequency.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

The single FIGURE of the drawing is a combined wiring diagram and schematic illustration of a travellingatent O ice wave tube frequency divider embodying the features of the present invention.

Referring now to the drawing, at 10 there is shown an envelope of a travelling-wave tube within which there is disposed at one end an electron gun section 12 for producing a beam of electrons 14 which travel along the axis of the tube, and a collector electrode or anode 16 disposed at the other end of the tube for intercepting the axial electron beam. The electron gun section includes cathode 13 and an accelerating electrode 15.

. A suitable longitudinal magnetic field coextensive with the beam path 14 may be provided by any suitable means (not shown) to focus and constrain the electron beam path. A pair of helices 20 and 22 or other suitable slow- Wave periodic structures are disposed in that order intermediate electron gun section 12 and anode 16 and are axially aligned with the electron beam path 14, the

energy absorber 24 and is provided with a tap 26 which contacts a first direct-current potential source 28. As

shown, the negative terminal of the potential source 28 is connected to cathode 13 and the positive terminal thereof is referenced to ground. The D.-C. potential applied to helix 20 may be varied with respect to cathode 13 by means of the tap 26. Similarly, the end terminal of helix 22 closest to collector electrode 16 is terminated in a matched energy absorbing load 30 and is provided with a tap 32 which contacts a second direct-current potential source 34-. The negative terminal of source 34 is also. connected to cathode 13 and the positive terminal is referenced to ground as shown. With this arrangement, the D.-C. potential applied to helix 22 may be varied with respect to cathode 13 by means of tap 32. Thus, the DC. voltage of each helix with respect to the cathode may be independently varied. The remaining proximal terminals 36 and 38 of helices 20 and 22 provide the input and output terminals, respectively, of the frequency divider as explained below. The length of both the helices 20 and 22 are adjusted to make both operate below start-oscillation at the desired operating current. Helix 20 may be operated well below startoscillation and the helix 22 may be operated very near start-oscillation so that the length of helix 20 is less critical than the length of helix 22. The only consequence of too low a gain in helix 20 is the requirement for more drive power of the input signal. Although not shown, it is to be understood of course that suitable waveguide coupling circuits well known in the art are to be used to couple energy into and out of the helices 20 and 22. Also, the external matching energy absorbing terminations 24 and 30 may be provided within the tube envelope by any suitable means well known in the art.

In the operation of the circuit shown in the drawing, the input frequency f which is to be divided is applied to input terminal 3 6 of helix 20 so that the input frequency travels along helix 20 opposite to the direction of the electron beam 14. The level of the signal f is arranged to drive the tube to saturation. The D.-C. voltage applied to helix 20 through tap 26 is such that the beam current through helix 20 is less than the startoscillation current and a value such that backward-wave interaction occurs between f and the electron beam passing therethrough. As is Well known, the signal travelling along helix 20 has a group velocity and phase velocity in opposite direction. Thus when the phase velocity is in the direction of the beam and in approximate synchronism therewith so that energy interchange takes place such as in a conventional forward travelling-wave tube, energy flows and increases backward toward the gun end of the tube. There is also an inherent regeneration from the energy fiow loops established by the oppositely directed movements of energy in the circuit and the beam. Thus, regenerative amplification is available below a critical or start-oscillation current which is voltage tuneable over a wide range because of the inherent dispersive character of the backward waves which may be supported by any geometrically periodic structure. readily be seen, therefore, that the beam leaving helix 20 and entering helix 22 will be modulated at frequency 11. Now, with helix 22 voltage tuned by means of tap 32 to a frequency f such that the difference frequency f f =f then it can be shown that the output frequency at terminal 38 of helix 22 is The beam current through helix 22 is preferably adjusted to provide a 20 db gain at the frequency f to which the helix 22 is voltage tuned by the tap 32. This requires a beam current approximately 90% of the start-oscillation current. As is well known, such a device operating in the backward wave mode is highly regenerative at the gain level of 20 db andprovides a highly selective gain vs. frequency characteristic. If a signal at frequency f is postulated as existing on helix 22, this signal will also modulate the beam and, due to the fact that these two frequencies f and f interact non-linearly with the electron beam while being propagated, the difference frequency 73-45 will also appear in the beam. The postulated signal is a noise signal at the frequency for maximum backward wave gain in helix 22. Even though it is very small, some diiference frequency signal at f f will be generated as a result of the inherent non-linearity of the beam, because the amplitude of the difference frequency is proportional to the product of the amplitudes of the two generating signals at and f If the difference frequency f f and f are the same frequency as hereinabove described, the signal at the difference fre quency will be amplified, feedback on itself via the usual backward wave mechanism and appear as f at the output terminal 38, that is the gun end, of helix 22. If the net gain of the system at frequency f is greater than unity, the tube will be self-sustaining, and, although not oscillating at f it will provide an output at this frequency. In the example described, the relationship f =f -f can only be satisfied when Thus the output at terminal 38 is under the conditions described. The postulation of a signal at frequency f =f f in helix 22 is a valid assumption and is analogous to the starting of an oscillator. However, it is to be understood that neither of the helices are operated as oscillators. It has been found that the loop gain is greater than unity for the mixed or difference signal at the same time that is less than unity for a signal that simply goes around the loop in the absence of input f If the input signal f is absent, no output at f will be provided. It has also been found that the input helix 20 may be operated in the forward wave mode with the signal applied at the gun end thereof and a matched load at terminal 36 without changing the operation of the circuit. Although the description has been shown in connection with a frequency division of /2 it is to be under- It can" TL em where n and m are integers. The amplitude of higher order components will necessarily be small and only those above a certain level will be obtainable. Since a higher order component will have an amplitudetha't is proportional to the amplitude of the signal at the frequency f to the n power and the amplitude of the postulated noise signal at f to the m power, a higher order component will not be self-starting unless the noise signal is large enough. It may therefore be necessary to start the system by injection of a signal at the higher order component frequency.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may. be made therein without departing from the invention, and: it is, therefore,

aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: I

1. A microwave frequency divider circuit comprising a travelling wave tube having an electron gun at one end thereof for directing an electron beam along a path and a collector anode at the other end thereof for intercepting said beam, means including a first discrete slow-wave circuit element in coupling relationship to said beam and having a signal input means at the end thereofnearer' said collector anode and an energy absorber at the other end thereof,- whereby when a signal of prescribed frequency and power is applied to said input means said beam is saturated and interacts with said signal in the backward-wave mode to modulate said beam at said frequency, a second discrete slow-wave circuit element intermediate said first slow-wave circuit element and said collector anode and in coupling relationship to said beam, said second slow-wave circuit element having a signal output means at the end thereof nearer said gun, and an energy absorber at the other end thereof, the beam current in coupling relation to said second-slow-wave circuit being below start-oscillation current, and means for voltage tuning the modulated beam interacting with said second slow-wave circuit element to produce a signal on said second slow-wave circuit element which is a submultiple of said prescribed input frequency, said submultiple frequency having a value such that it is equal to the beat frequency between said submultiple frequency and said prescribed frequency on said beam, said submultiple signal being derived from said signal output means.

2. A microwave frequency divider circuit comprising a travelling wave tube having an electron gun for directing an electron beam along a path and means for inter cepting said beam, means including a first discrete slowwave circuit element in coupling relationship to said beam and having a signal input means at the end thereof nearer said beam intercepting means and an energy absorber at the other end thereof, whereby when a signal of a prescribed frequency and power is applied to said input means said beam is saturated and interacts with said signal in the backward-wave mode to modulate said beam at said frequency, a second discrete slow-wave circuit element in coupling relationship with said beam intermediate said first slow-wave circuit element and said beam intercepting means, the beam current in coupling relationship to said second slow-wave circuit element being below start-oscillation current, said second slow-wave circuit element having a signal output means at the end thereof remote from said beam intercepting means and an energy absorber at the other end' thereof, means for voltage" tuning the modulated beam interacting with said second slow-wave circuit element to produce a signal thereon at a selected submultiple frequency of said predescribed frequency which interacts in the backward-wave mode with said beam, said selected frequency being such that the predescribed frequency signal and the submultiple frequency signal are mixed in said beam to produce a beat signal equal to the submultiple frequency and amplified by the backward-wave interaction of said second slow-wave circuit and the beam in coupling relationship thereto, the selected submultiple frequency signal being derived at the signal output means of the second slowwave circuit.

3. A microwave frequency divider circuit comprising a travelling-wave tube having an electron gun at one end thereof for directing an electron beam along a path and a collector anode at the other end thereof for intercepting said beam, means including a first discrete slow-wave circuit element in coupling relationship to said beam and having a signal input means at the end thereof nearer said electron gun and an energy absorber at the other end thereof, whereby when a signal of prescribed frequency and power is applied to said input means said beam is saturated and interacts with said signal in the forwardwave mode to modulate said beam at said frequency, a second discrete slow-wave circuit element intermediate said first slow-wave circuit element and said collector anode and in coupling relationship to said beam, said second slow-wave circuit element having a signal output means at the end thereof nearer said gun and an energy absorber at the other end thereof, the beam current in coupling relation to said second slow-wave circuit being below start oscillation current, and means for voltage tuning the modulated beam interacting with said second slow-wave circuit element to produce a signal on said second slow-wave circuit element which is a submultiple of said prescribed input frequency, said submultiple frequency having a value such that it is equal to the beat frequency between said submultiple frequency and said prescribed frequency on said beam, said submultiple signal being derived from said signal output means.

4. A microwave frequency divider circuit comprising a travelling wave tube having an electron gun at one end thereof for directing an electron beam along a path and a collector anode at the other end thereof for intercepting said beam, spaced first and second discrete helices intermediate said electron gun and said anode and in coupling relationship with said beam, the length of both helices being such that the beam current in coupling relation thereto is below start-oscillation current, said first helix having a signal input means at the end thereof nearer said collector anode and said second helix having asignal output means proximal the input. end of said first helix, the respective remaining terminals of said first and second helices being terminated by energy absorber means, first and second discrete voltage tuning means in circuit with said first and said second helices, respectively, said first voltage tuning means being adjusted such that when a signal of prescribed frequency and power is applied to the input of said first helix, said beam is saturated and interacts with said signal in the backward-wave mode to modulate said beam at said frequency, said second helix being responsive to said modulated beam and having its voltage tuning means adjusted such that there is producted thereon a signal at a selected submultiple frequency of said prescribed frequency which interacts in the backward-Wave mode with said beam, said submultiple frequency being such that the prescribed frequency signal and the submultiple frequency signal are mixed in said beam to produce a beat signal equal to the submultiple frequency and amplified by the backward-Wave interaction of said second helix and the beam in coupling relationship thereto, the selected submultiple frequency signal being derived at the signal output means of the second helix.

5. The microwave frequency divider in accordance with claim 4, wherein the beam current in coupling relationship to said second helix is of the start-oscillation current.

References Cited in the file of this patent UNITED STATES PATENTS 2,584,308 Tiley Feb. 5, 1952 2,748,268 Whinnery May 29, 1956 2,753,481 Ettenberg July 3, 1956 2,805,333 Waters Sept. 3, 1957 2,811,664 Kazan Oct. 29, 1957 2,814,756 Kenmoku Nov. 26, 1957 2,840,752 Cutler et al June 24, 1958 2,906,868 George Sept. 29, 1959 2,916,658 Currie Dec. 8, 1959 FOREIGN PATENTS 547,230 Canada Oct. 8, 1957 OTHER REFERENCES Proceedings of the IRE, November 1955, pages 1617 to 1631; The Cascade Backward-Wave Amplifier, by Currie and Whinnery. 

